This invention relates to the mounting of windows, and particularly to the mounting of windows that are subjected to large temperature changes during service.
Sensors and tracking devices operating in the visible and infrared wavelength bands are used in civilian and military applications. The optics and sensing elements of such sensors and trackers are of very high quality and easily damaged. They are therefore usually placed behind a protective window that is transparent to the radiation being sensed but protects the optics from damage due to hostile physical and chemical environmental effects, impacts, and the like.
The sensor is normally mounted inside a metallic or composite-material housing, such as a portion of the fuselage of an aircraft or a turret affixed to the aircraft. The window is a material that is transparent to the wavelength being utilized by the sensor or tracker. For visible light, the window may be a glass such as fused silica. The material of the window normally has a coefficient of thermal expansion quite different from that of the housing. In an example, a fused silica window has a coefficient of thermal expansion of 0.4.times.10.sup.-6 per .degree. C, and an aluminum alloy housing has a coefficient of thermal expansion of 21.times.10.sup.-6 per .degree. C. (The "coefficient of thermal expansion", as used herein, is the linear coefficient of thermal expansion.) In this typical example, the coefficient of thermal expansion of the window is much less than that of the housing, as is the case for many situations of practical interest.
The window is mounted to the housing by a mounting support. The nature of the mounting support depends upon the dimensions of the window. For small windows, on the order of a few inches in diameter, the strains and stresses caused by a difference in coefficients of thermal expansion between the window and the housing is of relatively little consequence. However, as the window is made larger, the strains and stresses caused by a difference in the coefficients of thermal expansion becomes an important consideration. The total distortion of a structure due to the differences in thermal expansion of the elements is proportional to the absolute dimensions of the structure, the difference in the coefficients of expansion, and the temperature range experienced as the structure is heated and cooled. High-power laser tracker systems now under development use windows that are 40-60 inches in diameter and operate over temperature ranges of 100.degree. C. or more, so that the total distortion of the mounting structure at the periphery of the window may be expected to be on the order of 0.1 inches.
When the structure is distorted by such large amounts, stresses are generated. For some applications, the stresses are readily sustained and are not troublesome. For a window application, on the other hand, the stresses, if transmitted into the window, cause the window itself to deform and possibly fail by shattering. The result of deformation of the window is severe distortion of the optical beam that passes through the window, greatly reducing the effectiveness of the sensor or tracking system.
There exist techniques for mounting the window into the housing that reduce the effects of the differences in coefficients of thermal expansion of the elements. In one, a sliding contact is effected between the window and the mounting support, which is sealed by O-rings or spring loaded seals. This approach has the drawbacks that the sliding contact has high friction forces, provides no moment relief, and has moderate leakage. In another approach, a flexible polymeric (typically rubber) sealing and mounting ring is bonded to both the window and the support structure. This approach has the drawbacks of leakage through the polymer that tends to contaminate the internal volume of the housing, outgassing into the internal volume of the housing, and variable mounting characteristics due to the change with temperature of the elasticity of the polymeric ring. Thus, the presently available approaches, while operable to some extent, are not fully satisfactory.
There is a need for an improved window structure operable with large windows and over a wide temperature range. The present invention fulfills this need, and further provides related advantages.